Heavy rain and northwesterly gales are expected to hit parts of the South island today, including Southland, Fiordland and Stewart Island.

The MetService has put a Strong Wind Watch in place for much of the South. It's forecast to get windy in Southland from 9pm tonight, until 9am tomorrow.

There's also a Heavy Rain Warning for Fiordland, around and to the north of George Sound, with up to 110mm expected to fall between 11pm tonight and 10am tomorrow.

"An active front, preceded by strengthening northwest winds, moves eastwards across the South Island during Tuesday and the North Island on Wednesday," the MetService update reads.

"This front is forecast to bring heavy rain to the west of the South Island and to the Otago and Canterbury headwaters. Gale to severe gale northwesterlies preceding the front are expected to affect the south and east of the South Island and the lower North Island. Heavy Rain and Strong Wind Warnings and Watches are in force for these areas."

Northwest winds may approach severe gale in exposed places, while heavy rain may cause streams and rivers to rise rapidly.

"Surface flooding and slips are also possible and driving conditions may be hazardous."

The public is advised to keep up to date with the latest forecasts in case warnings are upgraded, or further areas are added.